Many transmitters have been used to transmit a modulation signal with both amplitude modulation and frequency modulation. In remote control transmitters a signal has been sent by the transmitter to a remotely positioned receiver which receives the signal, if of proper carrier frequency and then if the modulation frequency is also in agreement with that of the receiver the receiver will energize a relay. This may be used in the remote control of a load at the receiver location, for example, a garage door operator to open and close a garage door. With the increasing numbers of radio controlled garage doors now in the United States there are many occasions when the number of codes provided by different carrier frequencies and modulation frequencies is insufficient. There may be interference between two closely adjacent receivers wherein a strong signal from a closely positioned transmitter can energize a relay in a receiver even when the carrier and modulation frequencies in the transmitter and receiver are not the same. A prior art solution was the provision of two modulation frequencies both modulating the carrier at the same time with the receiver on the same code having three different frequencies, namely the carrier and the two modulations frequencies in order to respond to such transmitted signal. Due to the limitations of frequencies available for the carrier frequency and frequencies available for the modulation frequencies which are not integral multiples, i.e. harmonics of one another, the number of possible codes is limited to two or three hundred. Often this is insufficient to prevent unwanted actuation of the incorrect receiver and thus the opening of a garage door other than the one intended.
One prior art solution was the utilization of neither pure amplitude nor frequency modulation but instead a transmission of a carrier wave turned on and off by a digital signal. First and second digital signals of a ratio of frequencies were utilized which were in phase agreement and were combined to provide either a short pulse or a long pulse so that a pulse train of varying pulse widths was used to modulate a carrier wave. A synchronization period of the carrier with an absence of pulses was then transmitted to indicate that this was the end of the message. The message was then repeated continuously by the transmitter. If the receiver was on the same code, that is, the same carrier frequency and the same sequence of narrow and wide pulses, then the receiver would be enabled and the remotely controlled load, e.g. a garage door opener would be actuated. These prior attempts at encoding transmitters are deficient in the number of codes to be transmitted. The digital control system has the deficiencies of relying upon the complexity of a shift register into which a particular digital pulse pattern was loaded and then serially supplied to an output so as to be provided as a modulation signal on the carrier wave. This system has no preamble pulse pattern to assure exclusivity of the transmitted signal.